Recently I encrypted my LG Tribute HD. I was doing something in recovery mode a couple days later, I accidentally hit the "wipe all data on phone" button. I pressed no. I rebooted my phone and all of the sudden secure startup was enabled and it had a password instead of a pattern. I tried a couple passwords but none of them worked. The attempts count only went down 1 time for some weird reason. I don't want to factory reset my phone. I just did a couple days ago, and I don't want to again.
Well I've ended in same place as you but with entering number in a dialer. White screen with only text box to enter password, and counter was going only to 29/30 tries left no matter how many times you enter password. This one?
One user said that this happened after he tried to clear dalvik/cache from stock recovery on LG (I think it was about K10). My guess that this action wipes the encryption key so when you try to boot after clearing it prompts you this, and you probably need to enter key which I'm sure you don't know for this user and me too factory reset was only way out.
See this and this and this
Q & A
